Implantable medical device with control of neural stimulation based on battery status

ABSTRACT

An implantable medical device is powered by a battery to deliver one or more therapies including at least one non-life-sustaining therapy such as neural stimulation for enhancing quality of life of a patient. When the battery approaches its end of life, the implantable medical device reduces power consumption of the neural stimulation (e.g., intensity of the neural stimulation) for extending the remaining battery life while maintaining a certain amount of therapeutic benefits for the patient. In one embodiment, the intensity of the neural stimulation is reduced in a tiered manner. In one embodiment in which the implantable medical device also delivers at least one life-sustaining cardiac stimulation therapy, the neural stimulation is disabled or adjusted to reduce its power consumption (e.g., intensity) while the intensity of the cardiac stimulation therapy is maintained when the battery is near its end of life.

CLAIM OF PRIORITY

This application is a continuation of U.S. application Ser. No.13/948,790, filed Jul. 23, 2013, now issued as U.S. Pat. No. 9,002,457,which claims the benefit of priority under 35 U.S.C. §119(e) of U.S.Provisional Patent Application Ser. No. 61/692,060, filed on Aug. 22,2012, 2010, each of which is herein incorporated by reference in itsentirety.

TECHNICAL FIELD

This document relates generally to implantable medical devices and moreparticularly to an implantable medical device providing for at leastneural stimulation and control of device functions including the neuralstimulation based on battery status.

BACKGROUND

Neural stimulation has been applied to modulate various physiologicfunctions and treat various diseases. One example is the modulation ofcardiac functions using autonomic modulation therapy (AMT) such as vagusnerve stimulation (VNS) therapy in a patient suffering heart failure ormyocardial infarction. The myocardium is innervated with sympathetic andparasympathetic nerves including the cardiac branches of the vagusnerve. Activities in the vagus nerve, including artificially appliedelectrical stimuli, modulate the heart rate and contractility (strengthof the myocardial contractions). Electrical stimulation applied to thevagus nerve is known to decrease the heart rate and the contractility,lengthening the systolic phase of a cardiac cycle, and shortening thediastolic phase of the cardiac cycle. This ability of VNS is utilized,for example, to control myocardial remodeling.

Batteries are used as energy sources for implantable medical devicesincluding those delivering neural stimulation. While the use of abattery allows a medical device to be totally implantable, without theneed of transcutaneous power transmission, the power consumption andlongevity of the medical device is limited by the capacity of thebattery. For example, many implantable medical devices providing forcardiac and/or neural stimulation treating cardiac disorders arelong-term treatments that may last up to the patient's lifetime. Whenthe battery of such an implantable medical device is no longer able toprovide sufficient energy for the operation of the device, the device isto be explanted and replaced. Because it may take weeks to months toarrange for the device replacement after such need is indicated based onthe energy state of the battery, there is a need to manage behavior ofthe implantable medical device during this period of time, when thebattery is near its end of life.

SUMMARY

An implantable medical device is powered by a battery to deliver one ormore therapies including at least one non-life-sustaining therapy suchas neural stimulation for enhancing quality of life of a patient. Whenthe battery approaches its end of life, the implantable medical devicereduces power consumption of the neural stimulation (e.g., intensity ofthe neural stimulation) for extending the remaining battery life whilemaintaining a certain amount of therapeutic benefits for the patient. Inone embodiment, the intensity of the neural stimulation is reduced in atiered manner. In one embodiment in which the implantable medical devicealso delivers at least one life-sustaining cardiac stimulation therapy,the neural stimulation is disabled or adjusted to reduce its powerconsumption (e.g., intensity) while the intensity of the cardiacstimulation therapy is maintained when the battery is near its end oflife.

In one embodiment, an implantable medical device can include a battery,a battery monitoring circuit, a neural stimulation circuit, and acontrol circuit. The battery monitoring circuit is configured to monitoran energy level of the battery and produce a battery status parameterindicative of the energy level. The neural stimulation circuit isconfigured to deliver neural stimulation for modulating neuralactivities. The control circuit can include a power controller and aneural stimulation controller. The power controller can be configured toset a current power mode of the implantable medical device to areduced-power operation mode of a plurality of power modes in responseto the battery status parameter indicating that the energy level hasfallen below an energy level threshold of a plurality of energy levelthresholds. The neural stimulation controller is configured to controlthe delivery of the neural stimulation using neural stimulationparameters and can be configured to adjust one or more parameters of theneural stimulation parameters within the reduced-power operation modesuch that a power consumption of the neural stimulation is reducedwithin the reduced-power operation mode according to a specified powerreduction schedule.

In one embodiment, a method for operating an implantable medical deviceis provided. An energy level of a battery of the implantable medicaldevice is monitored. A battery status parameter indicative of the energylevel is produced. Neural stimulation is delivered for modulating neuralactivities. The delivery of the neural stimulation is controlled usingneural stimulation parameters. A current power mode of the implantablemedical device can be set to a reduced-power operation mode of aplurality of power modes in response to the battery status parameterindicating that the energy level has fallen below an energy levelthreshold of a plurality of energy level thresholds. A power consumptionof the neural stimulation can be reduced within the reduced-poweroperation mode by adjusting one or more parameters of the neuralstimulation parameters according to a specified power reductionschedule.

In one embodiment, an implantable medical device includes a battery, abattery monitoring circuit, a plurality of functional modules, and acontrol circuit. The battery monitoring circuit is configured to monitoran energy level of the battery and produce a battery status parameterindicative of the energy level. The plurality of functional modules caninclude a neural stimulation module and a cardiac stimulation module.The neural stimulation module includes a neural stimulation circuitconfigured to deliver neural stimulation for modulating neuralactivities in the nervous system of a body. The cardiac stimulationmodule includes a cardiac stimulation circuit configured to deliver oneor more of pacing or defibrillation pulses to the heart of the body. Thecontrol circuit can be configured to control operation of eachfunctional module of the one or more functional modules and to reduce apower consumption of the neural stimulation in response to the batterystatus parameter indicating that the energy level has fallen below anenergy level threshold of a plurality of energy level thresholds.

In one embodiment, a method for operating an implantable medical devicein a body is provided. An energy level of a battery of the implantablemedical device is monitored. A battery status parameter indicative ofthe energy level is produced. Delivery of neural stimulation formodulating neural activities in the nervous system from the implantablemedical device and delivery of cardiac stimulation including one or moreof pacing or defibrillation pulses to the heart from the implantablemedical device can be controlled. Such control can include reducing apower consumption of the neural stimulation in response to the batterystatus parameter indicating that the energy level has fallen below anenergy level threshold of a plurality of energy level thresholds.

An example of reducing the power consumption of the neural stimulationincludes reducing the intensity of the neural stimulation. Anotherexample of reducing the power consumption of the neural stimulationincludes suspending delivery of the neural stimulation when a temporarypower-consumptive event is occurring, such as when the implantablemedical device is transmitting and/or receiving signals via telemetry.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects of the invention will be apparent to persons skilled in the artupon reading and understanding the following detailed description andviewing the drawings that form a part thereof. The scope of the presentinvention is defined by the appended claims and their legal equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate generally, by way of example, variousembodiments discussed in the present document. The drawings are forillustrative purposes only and may not be to scale.

FIG. 1 is an illustration of an embodiment of an implantable medicaldevice (IMD) system and portions of an environment in which the systemis used.

FIG. 2 is a block diagram illustrating an embodiment of an IMD.

FIG. 3 is a flow chart illustrating an embodiment of a method foroperating the IMD based on its battery status.

FIG. 4 is a flow chart illustrating an example of the method of FIG. 3performed with a specific example of battery status parameter values.

FIG. 5 is a flow chart illustrating another example of the method ofFIG. 3 performed with another specific example of battery statusparameter values.

FIG. 6 is a block diagram illustrating an embodiment of an IMD thatdelivers neural stimulation and controls the delivery of the neuralstimulation based on its battery status.

FIG. 7 is a flow chart illustrating an embodiment of a method foroperating the IMD of FIG. 6.

FIG. 8 is a block diagram illustrating an embodiment of an IMD thatdelivers neural stimulation and cardiac stimulation and controls thedelivery of the neural stimulation and the delivery of the cardiacstimulation based on its battery status.

FIG. 9 is a flow chart illustrating an embodiment of a method foroperating the IMD of FIG. 8.

FIG. 10 is a block diagram illustrating an embodiment of a neuralstimulation module of an IMD.

FIG. 11 is a block diagram illustrating an embodiment of a cardiacstimulation module of an IMD.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description provides examples,and the scope of the present invention is defined by the appended claimsand their legal equivalents.

It should be noted that references to “an”, “one”, or “various”embodiments in this disclosure are not necessarily to the sameembodiment, and such references contemplate more than one embodiment.

This document discusses a method and system for controlling operation ofan implantable medical device (IMD) based on the energy level of itsbattery (non-rechargeable or rechargeable battery). In variousembodiments, the IMD delivers at least one non-life-sustaining therapy,such as an autonomic modulation therapy (AMT) delivered to a patient toenhance the patient's quality of life. When the energy level indicatesthat the battery is approaching its end of life, the power consumptionof the non-life-sustaining therapy is reduced to extend the remaininglife of the battery while maintaining a certain level of therapeuticbenefits received by the patient. In various embodiments, in addition tothe non-life-sustaining therapy, the IMD delivers at least onelife-sustaining therapy such as a bradycardia therapy for apacemaker-dependent patient or a ventricular defibrillation therapy.When the energy level indicates that the battery is approaching its endof life, the intensity of the life-sustaining therapy is maintainedwhile the non-life-sustaining therapy is disabled or delivered at areduced power consumption.

In this document, neural stimulation includes stimulation targeted atmodulating neural activities, including stimulation that is applied toone or more nerves or natural sensors such as baroreceptors, and cardiacstimulation includes stimulation targeted at modulating cardiac rhythms,including pacing and defibrillation pulses that are delivered to theheart directly or through other body tissue. While neural stimulationand cardiac stimulation are discussed as examples of non-life-sustainingtherapy and life-sustaining therapy in this document, respectively, thepresent subject matter applies to an IMD delivering any one or morenon-life-sustaining therapies or an IMD delivering bothnon-life-sustaining and life-sustaining therapies. The present methodand system can be applied to provide sufficient power for eachlife-sustaining therapy while reducing the amount of power consumed byeach non-life-sustaining therapy as the battery of the IMD approachesits end of life. While IMDs are discussed as examples, the presentmethod and system can be applied to any battery-powered medical devices.

FIG. 1 is an illustration of an embodiment of an IMD system 100 andportions of an environment in which system 100 used. System 100 includesan implantable system 105, an external system 114, and a telemetry link112. Implantable system 105 includes an IMD 110 and is placed in apatient as illustrated in FIG. 1. External system 114 and IMD 110communicates via telemetry link 112.

IMD 110 is powered by a battery, such as a primary cell battery or arechargeable battery. In various embodiments, IMD 110 is a neuralsensing and/or stimulation device. One example of such a device includesan AMT device that delivers neural stimulation to the autonomic nervoussystem, such as a vagus nerve. The neural sensing and/or stimulationfunctions are adjustable based on the status of the battery. Forexample, when the energy level of the battery falls below a specifiedthreshold, the intensity of a non-life-sustaining neural stimulationtherapy is reduced or stopped, or other power consuming features of IMD110 (e.g. sensing not required by a life-sustaining therapy, telemetry,etc.) may be adjusted or shut off to reduce power consumption, to extendthe remaining battery life. In various embodiments, IMD 110 integrates acardiac rhythm management (CRM) device with a neural sensing and/orstimulation device. The CRM device senses cardiac electrical activitiesand/or delivers cardiac stimulation. Examples of the CRM device includepacemakers, cardioverter/defibrillators, combinedpacemaker-cardioverter/defibrillators, cardiac resynchronization therapy(CRT) devices, and cardiac remodeling control therapy (RCT) devices. Invarious embodiments, cardiac activities are sensed to indicate a needfor cardiac stimulation and/or to control the timing of pacing pulsedeliveries. In various embodiments, cardiac activities are sensed tocontrol the timing of neural stimulation pulse deliveries, such as tosynchronize neural stimulation to cardiac cycles. The neural sensingand/or stimulation functions and the cardiac sensing and/or stimulationfunctions can be adjustable based on the status of the battery. Forexample, when the energy level of the battery falls below a specifiedthreshold, the intensity of a non-life-sustaining neural stimulationtherapy is reduced or stopped for extending the remaining battery life,while the intensity of a life-sustaining cardiac stimulation therapyremains unadjusted for ensuring patient safety. Examples of thelife-sustaining cardiac stimulation therapy include ventriculardefibrillation therapy and bradycardia pacing therapy forpacemaker-dependent patients.

External system 114 receives and processes data transmitted from IMD 110and controls operation of IMD 110. In one embodiment, external system114 includes a programmer that allows a user such as a physician orother caregiver to monitor the patient wearing IMD 110 and program IMD110 through telemetry link 112. In another embodiment, external system114 includes a patient monitoring system that includes an externaldevice communicating with IMD 110 through telemetry link 112 and aremote device coupled to the external device via a telecommunicationnetwork to allow the user to monitor the patient and/or program IMD 110from a remote location.

Telemetry link 112 provides for communication between IMD 110 andexternal system 114. In one embodiment, telemetry link 112 is aninductive telemetry link. In an alternative embodiment, telemetry link112 is a far-field radio-frequency telemetry link. In one embodiment,telemetry link 112 includes both an inductive telemetry link and afar-field radio-frequency telemetry link. In other words, IMD 110 andexternal system 114 are configured to communicate with each other usinginductive and/or far-field radio-frequency telemetry. The communicationincludes data transmission from IMD 110 to external system 114,including, for example, transmitting the data representative of sensedcardiac, neural, and/or other physiological signals in real time,extracting the data representative of sensed cardiac, neural, and/orother physiological signals stored in IMD 110, and extracting dataindicating an operational status of IMD 110 (e.g., battery status). Thecommunication also includes data transmission from external system 114to IMD 110, including, for example, programming IMD 110 to produce thedata representative of the sensed cardiac, neural, and/or otherphysiological signals, programming IMD 110 to perform at least oneself-diagnostic test (such as for a device operational status),programming IMD 110 to deliver cardiac and/or neural stimulationtherapies, and programming IMD 110 for adjusting its operations based onthe status of the battery.

FIG. 2 is a block diagram illustrating an embodiment of an IMD 210,which represents an embodiment of IMD 110. IMD 210 includes a battery224, a battery monitoring circuit 226, a plurality of functional modules220, a telemetry circuit 228, and a control circuit 222.

Battery monitoring circuit 226 monitors battery 224 and produces abattery status parameter indicative of the energy level or state ofdepletion of battery 224. Examples of battery 224 include a primary cellbattery and a rechargeable battery. In one embodiment, batterymonitoring circuit 226 measures one or more battery parametersindicative of an energy level of battery 224. Examples of the one ormore battery parameters include a terminal voltage, a charging time, andcharge depletion parameter. The terminal voltage is the voltage acrossthe two terminals of battery 224. The charging time is a time intervalduring which a capacitor in the IMD 110 (such as a defibrillationcapacitor, if IMD 110 includes a defibrillator) is charged to aspecified energy level using battery 224. The charge depletion parameteris indicative of a cumulative charge depleted from battery 224. Anexample of measuring the charge depletion parameter is discussed in U.S.Pat. No. 7,194,308, entitled “SYSTEM AND METHOD FOR MONITORING ORREPORTING BATTERY STATUS OF IMPLANTABLE MEDICAL DEVICE”, assigned toCardiac Pacemakers, Inc., which is incorporated herein by reference inits entirety.

In various embodiments, the battery status parameter can be a parameterof the measured one or more battery parameters or a function of themeasured one or more battery parameters. In one embodiment, batterymonitoring circuit 226 analyzes the one or more battery parameters usingone or more energy level thresholds and produces the battery statusparameter using an outcome of the analysis. The battery status parameterhas values each indicating whether the energy level of the battery hasfallen below one of the one or more energy level thresholds. The one ormore energy level thresholds are specified based on a need or desire forinforming the patient or a user of system 100, such as the physician orother caregiver, of a need or necessity for replacement of IMD 110.Examples of the one or more energy level thresholds include “electivereplacement indication” (ERI) that indicates the need for scheduling forreplacement of IMD 110, “end of life” (EOL) that indicates the need forimmediate replacement of IMD 110, and “battery expired” (BEX) thatindicates that all functions of IMD 110, optionally with an exceptionfor telemetry, must be disabled as the remaining energy level of battery224 may not provide for proper performance of any of these functions. Inone embodiment, ERI and EOL are each chosen and programmed for a desiredremaining life expectancy of battery 224 (and hence IMD 110), such asabout 3 months from ERI to EOL, and about 3 months from EOL to BEX. Inone embodiment, IMD 110 has two energy level thresholds including EOLand BEX. In another embodiment, IMD 110 has three energy levelthresholds including ERI, EOL, and BEX. Other energy level threshold(s)may also be employed, as desired. In various embodiments in whichbattery 224 is a rechargeable battery, the energy level thresholds suchas ERI, EOL, and BEX may be referenced to an energy level at whichrecharging is necessary, rather than an end-of-life point as in the caseof a non-rechargeable primary cell battery.

Functional modules 220 are each powered by battery 224. In theillustrated embodiment, functional modules 220 include a neuralstimulation module 230, a cardiac stimulation module 232, and one ormore sensor modules 234. Neural stimulation module 230 includes a neuralstimulation circuit 236 that delivers neural stimulation to the nervoussystem of the patient and a neural stimulation controller 240 thatcontrols the delivery of the neural stimulation by executing a neuralstimulation algorithm for modulating electrical activities of thenervous system. Cardiac stimulation module 232 includes a cardiacstimulation circuit 238 that delivers cardiac stimulation to the heartof the patient and a cardiac stimulation controller 242 that controlsthe delivery of the cardiac stimulation by executing a cardiacstimulation algorithm for modulating electrical activities of the heart.Sensor module(s) 234 each include a sensor circuit configured to sense aphysiological signal. In various embodiments, one or more physiologicalsignals sensed by one or more sensor modules 234 are each used formonitoring the patient's conditions, controlling the delivery of theneural stimulation, and/or controlling the delivery of the cardiacstimulation. Functional modules 220 are illustrated in FIG. 2 by way ofexample but not by way of limitation. In various embodiments, IMD 210includes one or more functional modules such as neural stimulationmodule 230, cardiac stimulation module 232, one or more sensor modules234, and any combination of two or more of the modules 230, 232, or 234.In various embodiments, operation of each functional module offunctional modules 220 can be adjusted based on the status of battery224.

Telemetry circuit 228 receives incoming signals from external system 114and transmits outgoing signals to external system 114 via telemetry link112. In various embodiments, telemetry circuit 228 supports inductivetelemetry and/or far-filed radio-frequency telemetry. In one embodiment,operation of telemetry circuit 228 is not adjusted based on the statusof battery 224 such that communication with IMD 210 can be maintainedafter other functions of IMD 210 must be disabled as the life of thebattery expires. In another embodiment, operation of telemetry circuit228 may be limited based on the status of battery 224, such as to allowinductive telemetry only for conserving power when the battery is nearits end of life.

Control circuit 222 controls operation of each functional module offunctional modules 220. In the illustrated embodiment, control circuit222 includes a power controller 244 and portions of functional modules220 that control operation of various monitoring and therapeuticfunctions of IMD 210, including neural stimulation controller 240,cardiac stimulation controller 242, and circuitry for processing one ormore physiological signals sensed by sensor module(s) 234. Powercontroller 244 controls a power state of each functional module offunctional modules 220 according to a current power mode of a pluralityof power modes of IMD 210. The power modes each correspond to an energylevel threshold of the one or more energy level thresholds for battery224 and specify the power state of the each functional module. Eachpower state corresponds to an energy level threshold. Examples of thepower states for each functional module include a “normal” state inwhich the functional module is enabled, a “low-power” state in which thefunctional module is disabled, and a “reduced-power” state in which thefunctional module is enabled and adjusted for reducing power consumptionof the functional module. The low-power state may include an “off” or“no-power” state during which the disabled functional module iscompletely turned off and/or a “sleep” state during which the disabledfunctional module is turned off but maintained at a state allowingprompt reactivation when needed. Power controller 220 switches thecurrent power mode to a next power mode of the plurality of power modesusing the battery status parameter in response to a change in the valueof the battery status parameter. In various embodiments, the batterystatus parameters has discrete values each indicating whether the energylevel of the battery has fallen below one of the one or more energylevel thresholds. Thus, the change in the value of the battery statusparameter indicates that the energy level of the battery has fallenbelow one of the one or more energy level thresholds.

Functional modules 220 have an overall power consumption for the currentpower mode that is the sum of the power consumptions of all thefunctional modules when each functional module of functional module 220operates according to the current power mode. The overall powerconsumption of the next power mode is lower than the overall powerconsumption of the current power mode.

In various embodiments, IMD 210 includes a normal operation mode and oneor more reduced-power operation modes. The current power mode of IMD 210is set to the normal operation mode or one of the one or morereduced-power operation modes according to the value of the batterystatus parameter. Examples of the power states of each functional moduleof functional modules 220 during each operation mode of IMD 210 include:

-   -   the normal operation mode:        -   neural stimulation module 230: normal state;        -   cardiac stimulation module 232: normal state;        -   one or more sensor modules 234: normal state;    -   reduced-power operation mode 1 (ERI mode or EOL mode):        -   neural stimulation module 230: reduced-power state;        -   cardiac stimulation module 232: normal state;        -   one or more sensor modules 234: normal state or            reduced-power state    -   reduced-power operation mode 2 (ERI mode or EOL mode):        -   neural stimulation module 230: low-power state;        -   cardiac stimulation module 232: normal state;        -   one or more sensor modules 234: reduced-power state or            low-power state;    -   reduced-power operation mode 3 (ERI mode or EOL mode):        -   neural stimulation module 230: low-power state;        -   cardiac stimulation module 232: reduced-power state;        -   one or more sensor modules 234: low-power state;    -   reduced-power operation mode 4 (BEX mode):        -   neural stimulation module 230: low-power state;        -   cardiac stimulation module 232: low-power state;        -   one or more sensor modules 234: low-power state.            Power controller 244 sets the power state of each functional            module of functional modules 220 according to such operation            modes of IMD 210. The examples above are presented for            illustrative purposes only. In various embodiments, after            the energy level of battery has fallen below an energy level            threshold (such as ERI or EOL), indicating that the battery            is approaching its end of life, a functional module            performing only life-sustaining function(s) is set to the            normal state, while a functional module performing only            non-life-sustaining function(s) is set to the reduced-power            state or low-power state. Whether, when, and which power            state a non-life-sustaining functional module is set to is            determined by the therapeutic benefits and risk of losing            power needed for the life-sustaining function(s).

In various embodiments, the circuit of IMD 210, including its variousembodiments and elements discussed in this document, can be implementedusing a combination of hardware and software (including firmware). Invarious embodiments, control circuit 222 and battery monitoring circuit226, including its various embodiments and elements discussed in thisdocument, may be implemented using an application-specific circuitconstructed to perform one or more particular functions or ageneral-purpose circuit programmed to perform such function(s). Such ageneral-purpose circuit includes, but is not limited to, amicroprocessor or a portion thereof, a microcontroller or portionsthereof, and a programmable logic circuit or a portion thereof. Invarious embodiments, control circuit 222 and battery monitoring circuit226, including its various embodiments and elements discussed in thisdocument, can be programmed to perform the various methods discussed inthis document.

FIG. 3 is a flow chart illustrating an embodiment of a method 300 foroperating an IMD such as IMD 210 based on its battery status. In oneembodiment, control circuit 222 is programmed to perform method 300. TheIMD has a plurality of power modes including the normal operation modeand the one or more reduced-power operation modes. The IMD has one ormore functional modules each performing a monitoring or therapeuticfunction and a control circuit. The control circuit adjusts operation ofat least one functional module of the one or more functional modules toreduce the overall power consumption of the IMD during each of the oneor more reduced-power operation modes. In one embodiment, the overallpower consumption is reduced in a tiered manner within a reduced-poweroperation mode.

At 302, the IMD is implanted into the patient. At 304, upon theimplantation of the device, the current power mode of the IMD is set tothe normal operation mode (or kept at the normal operation mode if sucha mode is preset before the implantation). At 306, the battery of theIMD is monitored to produce the battery status parameter. If the batterystatus parameter indicates that the energy level has fallen below afirst energy level threshold at 308, the current power mode is switchedto the first reduced-power operation mode at 310. At 312, the battery iscontinued to be monitored during the first reduced-power operation mode.If the battery status parameter indicates that the energy level hasfallen below the next energy level threshold at 314, the current powermode is switched to the next reduced-power operation mode at 316. If thenext energy level threshold indicates that the battery has expired(i.e., about totally depleted) at 318, the IMD needs to be immediatelyexplanted and replaced at 320, if determined necessary for the patient.Otherwise, steps 312, 314, 316, and 318 are repeated until the batteryhas expired.

FIG. 4 is a flow chart illustrating a method 400 being an example ofmethod 300. In one embodiment, control circuit 222 and batterymonitoring circuit 226 are programmed to perform method 400.

At 402, the IMD is implanted into the patient. At 404, upon theimplantation of the device, the current power mode of the IMD is set to(or kept at) the normal operation mode. At 406, the battery of the IMDis monitored to produce the battery status parameter. If the batterystatus parameter indicates that ERI has been reached at 408, the currentpower mode is switched to the ERI operation mode at 410. At 412, thebattery is continued to be monitored to produce the battery statusparameter during the ERI operation mode. If the battery status parameterindicates that EOL has been reached at 414, the current power mode isswitched to the EOL mode at 416. At 418, the battery is continued to bemonitored to produce the battery status parameter during the EOLoperation mode. If the battery status parameter indicates that BEX hasbeen reached at 420, the current power mode is switched to the BEX modeat 422. During the BEX mode, the IMD is to be immediately explanted andreplaced at 424, if determined to be necessary for the patient.

FIG. 5 is a flow chart illustrating a method 500 being another exampleof method 300. In one embodiment, control circuit 222 and batterymonitoring circuit 226 are programmed to perform method 500.

At 502, the IMD is implanted into the patient. At 504, upon theimplantation of the device, the current power mode of the IMD is set tothe normal operation mode. At 506, the battery of the IMD is monitoredto produce the battery status parameter. If the battery status parameterindicates that EOL has been reached at 508, the current power mode isswitched to the EOL operation mode at 510. At 512, the battery iscontinued to be monitored to produce the battery status parameter duringthe EOL operation mode. If the battery status parameter indicates thatBEX has been reached at 514, the current power mode is switched to theBEX mode at 516. During the BEX mode, the IMD is to be immediatelyexplanted and replaced at 518, if determined to be necessary for thepatient.

FIG. 6 is a block diagram illustrating an embodiment of an IMD 610 thatdelivers neural stimulation and controls the delivery of the neuralstimulation based on its battery status. IMD 610 represents anembodiment of IMD 210 and is a stand-alone neural stimulator (i.e.,without circuitry for cardiac stimulation). IMD 610 includes battery224, battery monitory circuit 226, neural stimulation circuit 236, and acontrol circuit 622. Battery monitoring circuit 226 monitors the energylevel of battery 224 and produces the battery status parameterindicative of the energy level. Neural stimulation circuit 236 deliversthe neural stimulation to the nervous system of the patient. Controlcircuit 622 represents an embodiment of control circuit 222 and includesa power controller 644 and a neural stimulation controller 640. Powercontroller 644 sets the current power mode of IMD 610 to a reduced-poweroperation mode of a plurality of power modes of IMD 610 in response tothe battery status parameter indicating that the energy level has fallenbelow an energy level threshold of a plurality of energy levelthresholds. Neural stimulation controller 640 controls the delivery ofthe neural stimulation using neural stimulation parameters and adjustsone or more parameters of the neural stimulation parameters within thereduced-power operation mode such that, for example, an intensity of theneural stimulation is reduced in a tiered manner within thereduced-power operation mode according to a specified power reductionschedule.

FIG. 7 is a flow chart illustrating an embodiment of a method 700 foroperating an IMD such as IMD 610. At 702, neural stimulation isdelivered from the IMD. At 704, energy level of the battery of the IMDis monitored. At 706, a battery status parameter indicative of theenergy level is produced. In response to the battery status parameterindicating that the energy level has fallen below an energy levelthreshold of a plurality of energy level thresholds at 708, the currentpower mode of the IMD is set to a reduced-power operation mode of aplurality of power modes of the IMD at 710. At 712, intensity of theneural stimulation is reduced in a tiered manner within thereduced-power operation mode by adjusting one or more parameters of theneural stimulation parameters according to a specified power reductionschedule.

FIG. 8 is a block diagram illustrating an embodiment of an IMD 810 thatdelivers neural stimulation and cardiac stimulation and controls thedelivery of the neural stimulation and the delivery of the cardiacstimulation based on its battery status. IMD 810 represents anotherembodiment of IMD 210 and is a combined neural and cardiac stimulationdevice. IMD 810 includes battery 224, battery monitoring circuit 226, aplurality of functional modules 820, and a control circuit 822. Batterymonitoring circuit 226 monitors the energy level of battery 224 andproduces a battery status parameter indicative of the energy level.Functional modules 820 include a neural stimulation module 830 and acardiac stimulation module 832. Neural stimulation module 830 representsan embodiment of neural stimulation module 230 and includes neuralstimulation circuit 236 that delivers the neural stimulation to thenervous system of the patient. Cardiac stimulation module 832 representsan embodiment of cardiac stimulation module 232 and includes cardiacstimulation circuit 238 that deliver the cardiac stimulation to theheart of the patient. Control circuit 822 represents an embodiment ofcontrol circuit 222 and controls operation of each functional module offunctional module 820. In response to the battery status parameterindicating that the energy level has fallen below an energy levelthreshold of a plurality of energy level thresholds, control circuit 822reduces intensity of the neural stimulation while maintaining intensityof the cardiac stimulation.

FIG. 9 is a flow chart illustrating an embodiment of a method 900 foroperating an IMD such as IMD 810. At 902, neural stimulation and cardiacstimulation are delivered from the IMD. At 904, the energy level of thebattery of the IMD is monitored. At 906, a battery status parameterindicative of the energy level is produced. In response to the batterystatus parameter indicating that the energy level has fallen below anenergy level threshold of a plurality of energy level thresholds at 908,intensity of the neural stimulation is reduced while intensity of thecardiac stimulation is maintained at 910.

FIG. 10 is a block diagram illustrating an embodiment of a neuralstimulation module 1030, which represents an embodiment of neuralstimulation module 230. In the illustrated embodiment, neuralstimulation module 1030 includes a neural sensing circuit 1050, neuralstimulation circuit 236, and a neural stimulation controller 1040.Neural sensing circuit 1050 senses one or more neural signals formonitoring the patient's conditions and/or controlling the delivery ofthe neural stimulation. In various other embodiments, neural stimulationmodule 1030 may not include neural sensing circuit 1050. Neuralstimulation circuit 236 delivers the neural stimulation to the nervoussystem of the patient. In one embodiment, the neural stimulationincludes electrical pulses. In one embodiment, the neural stimulation isa non-life-sustaining therapy that is applied to enhance the patient'squality of life.

Neural stimulation controller 1040 represents an embodiment of neuralstimulation controller 640 and may be part of a control circuit such ascontrol circuit 222, 622, or 822, which is part of an IMD such as IMD210, 610, and 810, respectively. The IMD has power modes each specifyinga power state of neural stimulation module 1030. The control circuitincludes a power controller that switches the current power mode of theIMD to the next power mode to reduce a power consumption required fordelivering the neural stimulation. Neural stimulation controller 1040controls the delivery of the neural stimulation by executing a neuralstimulation algorithm for modulating electrical activities of thenervous system using the neural stimulation parameters according to thecurrent power mode.

Neural stimulation controller 1040 includes a neural stimulationparameter 1052 and an adjustment timer 1054. Neural stimulationparameter adjuster 1052 adjusts one or more parameters of the neuralstimulation parameters according to the current power mode. Adjustmenttimer 1054 controls timing of adjustment of the neural stimulationparameters within the current power mode such that an intensity of theneural stimulation (and hence a power consumption of neural stimulationmodule 1030) is reduced in a tiered manner within the current power modeaccording to a specified power reduction schedule. The power reductionschedule specifies a plurality of time intervals and one or moreparameters of the neural stimulation parameters to be adjusted uponexpiration of each time interval of the plurality of time intervals,such that the power consumption of the neural stimulation module isreduced upon expiration of each time interval of the plurality of timeintervals. In other words, adjustment timer 1054 times a plurality oftime intervals, and neural stimulation parameter adjuster 1052 adjustsone or more parameters of the neural stimulation parameters uponexpiration of each time interval of the plurality of time intervals. Thetime intervals and the one or more parameters to be adjusted uponexpiration of each time interval are predetermined by balancing theamount of reduction in the power consumption of neural stimulationmodule 1030 (and hence the IMD) and reduction of therapeutic effects ofthe neural stimulation.

In an example, the neural stimulation algorithm is an AMT algorithmincluding stimulation parameters selected to modulate one or morecardiovascular functions by delivering electrical pulses to one or moretarget nerves. Examples of the stimulation parameters include pulseamplitude, pulse width, pulse frequency (or inter-pulse interval),periodic dose, and duty cycle. The pulse amplitude and pulse width areselected to ensure that each pulse elicits an action potential in thetarget nerve. The periodic dose is a time interval during which apatient is treated with neural stimulation for each predeterminedperiod. In one embodiment, the predetermined period is a day, and theperiodic dose is a daily dose. The duty cycle is the duty cycle of theneural stimulation during the time interval of the period dose. Forexample, if the patient is to receive a neural stimulation therapy fortwo hours each day, the periodic dose is 2 hours/day (or the daily doseis 2 hours). If the neural stimulation during those two hours isdelivered intermittently with alternating on- and off-periods, the dutycycle is the ratio of the on-period to the sum of the on-period and theoff-period.

Examples of the stimulation parameters when the power state of theneural stimulation module is switched from the normal state to areduced-power state, or from a reduced-power state to the nextreduced-power state includes:

-   -   pulse amplitude:        -   reduced to a level exceeding a laryngeal vibration threshold            by a specified margin, where laryngeal vibration is used to            indicate neural capture, and the laryngeal vibration            threshold is used as a neural capture threshold;        -   reduced by a specified value (e.g., 1 mA) or specified            percentage (e.g., 50%)        -   reduced by a specified value or percentage but subject to a            minimum level exceeding the laryngeal vibration threshold by            the specified margin;    -   pulse width;        -   reduced by a specified amount (e.g., 300 to 200, or 150, or            120 μs);    -   duty cycle:        -   reduced by a specified percentage (e.g., about 50%, or 17%            to 8%);    -   periodic dose:        -   reduced such as daily dose specified by the time interval            during which the neural stimulation is delivered (e.g.,            reduced from continuous delivery to periodic delivery, or            daily dose reduced from 24 hours to 12 or 8 hours);    -   pulse frequency:        -   reduced by a specified percentage (e.g., 25%: 20 to 15 Hz;            50%: 20 to 10 Hz).

In an example, the power reduction schedule specifies that the dutycycle is reduced upon switching to the next power mode (ERI or EOLmode), the periodic dose is reduced upon expiration of the first timeinterval of the plurality of time intervals, and the pulse amplitude isreduced upon expiration of the second time interval of the plurality oftime intervals.

FIG. 11 is a block diagram illustrating an embodiment of a cardiacstimulation module 1132, which represents an embodiment of cardiacstimulation module 232. In the illustrated embodiment, cardiacstimulation module 1132 includes a cardiac sensing circuit 1160, acardiac stimulation circuit 1138, and a cardiac stimulation controller1142. Cardiac sensing circuit 1160 senses one or more cardiac signalsfor monitoring the patient's conditions and/or controlling the deliveryof the cardiac stimulation. In various other embodiments, the one ormore cardiac signals may also be used to control the delivery of theneural stimulation. Cardiac stimulation circuit 1138 represents anembodiment of cardiac stimulation circuit 238 and delivers cardiacstimulation to the heart of the patient. In the illustrated embodiment,cardiac stimulation circuit 1138 includes a pacing circuit 1156 and adefibrillation circuit 1158. Pacing circuit 1156 delivers pacing pulsesto the heart. Defibrillation circuit 1158 delivers defibrillation pulsesto the heart. In various embodiments, the cardiac stimulation is alife-sustaining therapy. For example, pacing circuit 1156 may deliveranti-bradycardia pacing pulses to the heart of a pacemaker-dependentpatient, and defibrillation circuit 1158 delivers ventriculardefibrillation pulses to the heart if the patient suffers fromventricular fibrillations.

Cardiac stimulation controller 1142 represents an embodiment of cardiacstimulation controller 242 and may be part of a control circuit such ascontrol circuit 222 or 822, which is part of an IMD such as IMD 210 and810, respectively. The IMD has power modes each specifying a power stateof cardiac stimulation module 1132 in addition to the power state ofneural stimulation module 1030. The control circuit includes a powercontroller that adjusts the power state of neural stimulation module1030 to reduce its power consumption without adjusting the power stateof cardiac stimulation module 1132 in response to the current power modebeing switched to the next power mode. Cardiac stimulation controller1142 controls the delivery of the cardiac stimulation by executing acardiac stimulation algorithm for modulating electrical activities ofthe heart using cardiac stimulation parameters according to the currentpower mode. In various embodiments, cardiac stimulation controller 1142adjusts one or more parameters to reduce intensity of the cardiacstimulation only when the cardiac stimulation includes at least onenon-life-sustaining therapy. When the cardiac stimulation is consideredlife-sustaining, the available intensity of the cardiac stimulation ismaintained in the reduced-power operation modes until the battery isunable to supply for such intensity.

In one embodiment, sensor module(s) 234 as illustrated in FIG. 2 includean activity sensor, such as an accelerometer, that senses an activitysignal indicative of a physical activity level. Cardiac stimulationcontroller 1142 may use the sensed activity level to control thedelivery of pacing pulses. In various embodiments, sensor module(s) 234as illustrated in FIG. 2 may include any sensors used for monitoring ofthe patient, control of the neural stimulation by neural stimulationcontroller 1040, and/or control of cardiac stimulation by cardiacstimulation controller 1142. Examples of such sensors include theactivity sensor, a sensor for leadless sensing of cardiac activities, animpedance sensor for sensing cervical impedance plethysmogram, and/or arespiratory sensor for sensing tidal volume and/or minute ventilation.

In various embodiments, IMD 210 includes neural stimulation module 230and sensor module(s) 234. The power modes of IMD 210 each specify apower state of each sensor of sensor module(s) 234, in addition to thepower state of neural stimulation module 230. In various otherembodiments, IMD 210 includes neural stimulation module 230, cardiacstimulation module 232, and sensor module(s) 234. The power modes of IMD210 each specify a power state of each sensor of sensor module(s) 234,in addition to the power state of neural stimulation module 230 and thepower state of cardiac stimulation module 232. In one embodiment, powercontroller 244 disables at least one sensor module of sensor module(s)234 in response to the current power mode being switched to the nextpower mode. In one embodiment, power controller 244 adjusts the powerstate of neural stimulation module 230 to reduce its power consumptionand adjusts the power state of at least one sensor module of sensormodule(s) 234 to reduce the power consumption of sensor module(s) 234without adjusting the power state of cardiac stimulation module 232 inresponse to the current power mode being switched to the next powermode. For example, power controller 244 may set the power state ofcardiac stimulation module 232 to the normal state, set the power stateof neural stimulation module 230 to the reduced-power state or thelow-power state, and set the power state of at least one sensor moduleof sensor module(s) 234 to the reduced-power state or the low-powerstate during a reduced-power mode in response to the current power modebeing switched to the next power mode.

In response to the current power mode being switched to the next powermode, at least one sensor module of sensor module(s) 234 may bedisabled, and consequently control of the neural stimulation and/orcardiac stimulation may change from closed-loop to open-loop. If thedisabled sensor module includes a sensor used by neural stimulationcontroller 1040 to control the delivery of the neural stimulation, andthe neural stimulation continues to be delivered, neural stimulationcontroller 1040 switches the control of the delivery from closed-loop(using the sensor) to open-loop. If the disabled sensor module includesa sensor used by cardiac stimulation controller 1142 to control thedelivery of the cardiac stimulation, and the cardiac stimulationcontinues to be delivered, cardiac stimulation controller 1142 switchesthe control of the delivery from closed-loop (using the sensor) toopen-loop.

In one embodiment, when the current power mode of IMD 210 is in one ofthe one or more reduced-power operation modes, telemetry circuit 228 isset to a reduced-power state in which inductive telemetry is supportedwhile far-field radio-frequency telemetry is disabled. In anotherembodiment, neural stimulation module 230 and/or cardiac stimulationmodule 232 are each set to the reduced-power state or low-power stateduring a telemetry session, i.e., when telemetry circuit 228 istransmitting and/or receiving signals. In various embodiments, thelongevity of IMD 210 can be increased by suspending or reducingintensity of one or more non-life-sustaining therapies during atelemetry session. In one embodiment, the one or morenon-life-sustaining therapies are suspended or adjusted for reducedintensity when the current power mode of IMD 210 is in one of the one ormore reduced-power operation modes. In one embodiment, the one or morenon-life-sustaining therapies are suspended or adjusted for reducedintensity during a far-field radio-frequency telemetry session i.e.,when telemetry circuit 228 is transmitting and/or receiving signals viafar-field radio-frequency telemetry. In one embodiment, the one or morenon-life-sustaining therapies include one or more neural stimulationtherapies. When neural stimulation module 230 is set to thereduced-power or low-power state, battery 224 can be run to a lowercharge state while still providing IMD 210 the ability to communicatewith external system 114 via telemetry.

In various embodiments, when the current power mode of IMD 210 is set toone of the one or more reduced-power operation modes, sensed cardiac andother physiological signals may no longer be stored in IMD 210. Invarious embodiments, when the current power mode of IMD 210 is set toone of the one more reduced-power operation modes, daily wake-upsessions of IMD 210 and/or communication sessions of IMD 210 withexternal system 114 may be reduced or suspended. In general, any portionof IMD 210 that can be safely set to a reduced-power state may be set tothe reduced-power state when the need for conserving battery energyarises, as determined by those skilled in the art.

It is to be understood that the above detailed description is intendedto be illustrative, and not restrictive. Other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. An implantable medical device, comprising: abattery; a battery monitoring circuit coupled to the battery, thebattery monitoring circuit configured to measure a battery parameterindicative of an energy level of the battery; a neural stimulationcircuit configured to deliver neural stimulation for modulating neuralactivities; a cardiac stimulation circuit configured to deliver cardiacstimulation including one or more of cardiac pacing or defibrillation;and a control circuit coupled to the battery monitoring circuit, theneural stimulation circuit, and the cardiac stimulation circuit, thecontrol circuit configured to: set a current power mode of theimplantable medical device to a first predetermined reduced-poweroperation mode of a plurality of predetermined power modes in responseto the battery parameter indicating that the energy level of the batteryhas fallen below a first energy level threshold; reduce a powerconsumption required for delivering the neural stimulation withoutreducing a power consumption required for delivering the cardiacstimulation during the first predetermined reduced-power operation mode;set a current power mode of the implantable medical device to a secondpredetermined reduced-power operation mode of the plurality ofpredetermined power modes in response to the battery parameterindicating that the energy level of the battery has fallen below asecond energy level threshold; and reduce the power consumption requiredfor delivering the neural stimulation and the power consumption requiredfor delivering the cardiac stimulation during the second predeterminedreduced-power operation mode.
 2. The implantable medical device of claim1, wherein the neural stimulation is a non-life-sustaining therapy, andthe cardiac therapy is a life-sustaining therapy.
 3. The implantablemedical device of claim 2, wherein the neural stimulation comprises anautonomic modulation therapy.
 4. The implantable medical device of claim3, wherein the control circuit is configured to reduce the powerconsumption required for delivering the neural stimulation by reducingan intensity of the neural stimulation.
 5. The implantable medicaldevice of claim 4, wherein the neural stimulation circuit is configuredto deliver electrical pulses, and the control circuit is configured toreduce the intensity of the neural stimulation by adjusting one or moreof a duty cycle, a periodic dose, a pulse amplitude, a pulse width, or apulse frequency.
 6. The implantable medical device of claim 3, furthercomprising a sensor configured to allow the control circuit to performclosed-loop control of the delivery of the neural stimulation during apredetermined normal operation mode of the plurality of predeterminedpower modes, and wherein the control circuit is configured to reduce thepower consumption required for delivering the neural stimulation bydisabling the sensor and performing open-loop control of the delivery ofthe neural stimulation.
 7. The implantable medical device of claim 3,further comprising a telemetry circuit coupled to the control circuitand configured to transmit and receive signals, and wherein the controlcircuit is configured to reduce the power consumption required fordelivering the neural stimulation by reducing an intensity of the neuralstimulation during a telemetry session during which the telemetrycircuit transmits and receives the signals.
 8. The implantable medicaldevice of claim 3, wherein the cardiac stimulation comprises one or moreof a bradycardia therapy or a ventricular defibrillation therapy.
 9. Theimplantable medical device of claim 1, wherein the battery has twoterminals, and the battery monitoring circuit is configured to measure aterminal voltage as the battery parameter, the terminal voltage being avoltage across the two terminals of the battery.
 10. The implantablemedical device of claim 1, wherein the battery monitoring circuit isconfigured to measure a charge depletion parameter indicative of acumulative charge depleted from the battery.
 11. A method for operatingan implantable medical device, comprising: monitoring an energy level ofa battery of the implantable medical device; producing a batteryparameter indicative of the energy level; delivering neural stimulationfor modulating neural activities; delivering cardiac stimulationincluding one or more of cardiac pacing or defibrillation; setting acurrent power mode of the implantable medical device to a firstpredetermined reduced-power operation mode of a plurality ofpredetermined power modes in response to the battery parameterindicating that the energy level has fallen below a first energy levelthreshold; reducing a power consumption required for delivering theneural stimulation without reducing a power consumption required fordelivering the cardiac stimulation during the first predeterminedreduced-power operation mode; setting the current power mode of theimplantable medical device to a second predetermined reduced-poweroperation mode of the plurality of predetermined power modes in responseto the battery parameter indicating that the energy level has fallenbelow a second energy level threshold; and reducing the powerconsumption required for delivering the neural stimulation and the powerconsumption required for delivering the cardiac stimulation during thesecond predetermined reduced-power operation mode.
 12. The method ofclaim 11, wherein delivering the neural stimulation comprises deliveringa non-life-sustaining autonomic modulation therapy.
 13. The method ofclaim 12, wherein delivering the cardiac stimulation comprisesdelivering one or more of a life-sustaining bradycardia pacing therapyor a life-sustaining ventricular defibrillation therapy.
 14. The methodof claim 13, wherein reducing the power consumption required fordelivering the neural stimulation comprises reducing an intensity of theneural stimulation.
 15. The method of claim 14, wherein delivering theneural stimulation comprises delivering electrical pulses, and reducingthe intensity of the neural stimulation comprises adjusting a dutycycle.
 16. The method of claim 14, wherein delivering the neuralstimulation comprises delivering electrical pulses, and reducing theintensity of the neural stimulation comprises adjusting a periodic dose.17. The method of claim 14, wherein delivering the neural stimulationcomprises delivering electrical pulses, and reducing the intensity ofthe neural stimulation comprises adjusting one or more of a pulseamplitude or a pulse width.
 18. The method of claim 14, whereindelivering the neural stimulation comprises delivering electricalpulses, and reducing the intensity of the neural stimulation comprisesadjusting a pulse frequency.
 19. The method of claim 11, whereinreducing the power consumption required for delivering the neuralstimulation comprises changing control of the delivery of the neuralstimulation from closed-loop control to open-loop control.
 20. Themethod of claim 11, further comprising transmitting signals to and fromthe implantable medical device using telemetry, and wherein reducing thepower consumption required for delivering the neural stimulationcomprises reducing an intensity of the neural stimulation while thesignals are transmitted to or from the implantable medical device usingthe telemetry.